The present invention relates generally to radio frequency (RF) filters and more particularly to an RF filter having a temperature compensated ceramic resonator adaptable for use in antenna combiners coupling a plurality of RF transmitters to a single antenna.
In order to combine a number of RF transmitters, the RF signals from each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. RF filters of the air-filled cavity type may be utilized to provide isolation between the RF transmitters. Each such cavity filter is tuned to pass only the RF signal from the transmitter to which it is connected, each RF transmitter producing a different frequency RF signal. A conventional mechanism utilized to temperature compensate such cavity filters is described in U.S. Pat. No. 4,024,481. However, such air-filled cavity filters are both expensive and relatively large in size such that these cavity filters consume an inordinate amount of precious space at remote antenna sites located on top of buildings and mountains.
The size of such RF filters can be reduced by utilizing a ceramic resonator. One such filter utilizing a ceramic resonator is described in U.S. Pat. No. 4,241,322. Although providing a more compact filter, the ceramic resonator in such a filter is not temperature compensated for temperature changes in the ceramic due to RF power dissipation in the ceramic and therefore can experience large shifts in resonant frequency due to RF power dissipation and the fact that the ceramic cannot be made with exactly zero temperature coefficient. Another filter described in U.S. Pat. No. 4,019,161 utilizes conventional mechanisms to temperature compensate a ceramic resonator mounted on a micro-integrated circuit substrate, but does not provide for dissipation of heat in the ceramic resonator.